Production of oil and gas from offshore fields has created many unique engineering challenges. One of these challenges is dealing with effects of currents on marine elements. Such marine elements are employed in a variety of applications, including, e.g., subsea pipelines; drilling, production, import and export risers; tendons for tension leg platforms; legs for traditional fixed and for compliant platforms; other mooring elements for deepwater platforms; and, the hull structure for spar type structures. These currents may cause vortexes to shed from the sides of the marine elements, inducing vibrations that can lead to the failure of the marine elements or their supports.
Deepwater production risers, drilling risers, platform export risers, import risers bringing in production from satellite wells, tendons for tension leg platforms, and other conduits for produced fluids and deepwater mooring elements formed from tubular goods are typical of applications that may have vibration problems. Subsea pipelines traversing valleys on the ocean floor for extended, unsupported lengths and spar hulls moored at the end of long tethers and/or mooring lines provide additional examples.
When these types of structures, such as a cylinder or a pipe, experience a current in a flowing fluid environment, it is possible for the structure to experience vortex-induced vibrations (VIV). These vibrations are caused by oscillating dynamic forces on the surface which can cause substantial vibrations of the structure, especially if the forcing frequency is at or near a structural natural frequency.
Drilling for and/or producing hydrocarbons or the like from subterranean deposits which exist under a body of water exposes underwater drilling and production equipment to water currents and the possibility of VIV.
Risers are discussed in this patent document as a non-exclusive example of an aquatic structure subject to VIV (others would include tubulars, rods, bars, beams, cables, etc). A riser system may be used for establishing fluid communication between the surface and the bottom of a water body. The principal purpose of the riser is to provide a fluid flow path between a drilling vessel and a well bore and to guide a drill string to the well bore.
A typical riser system normally consists of one or more fluid-conducting conduits which extend from the surface to a structure (e.g., wellhead) on the bottom of a water body. For example, in the drilling of a submerged well, a drilling riser usually consists of a main conduit through which the drill string is lowered and through which the drilling mud is circulated from the lower end of the drill string back to the surface. In addition to the main conduit, it is conventional to provide auxiliary conduits, e.g., choke and kill lines, etc., which extend parallel to and are carried by the main conduit.
The magnitude of the stresses on the riser pipe is generally a function of and increases with the velocity of the water current passing these structures and the length of the structure.
There are generally two kinds of current-induced stresses exerted on structures in flowing fluid environments. The first kind of stress is caused by vortex-induced alternating forces that vibrate the structure (“vortex-induced vibrations”) in a direction perpendicular to the direction of the current. When fluid flows past the structure, vortices are alternately shed from each side of the structure. This produces a fluctuating force on the structure transverse to the current. If the frequency of this harmonic load is near the resonant frequency of the structure, large vibrations transverse to the current can occur. These vibrations can, depending on the stiffness and the strength of the structure and any welds, lead to unacceptably short fatigue lives. In fact, stresses caused by high current conditions in marine environments have been known to cause structures such as risers or pipes to break apart and fall to the ocean floor.
The second type of stress is caused by drag forces which push the structure in the direction of the current due to the structure's resistance to fluid flow. The drag forces may be amplified by vortex induced vibrations of the structure. For instance, a riser pipe that is vibrating due to vortex shedding will disrupt the flow of water around it more than a stationary riser. This may result in more energy transfer from the current to the riser, and hence more drag.
Some devices used to reduce vibrations caused by vortex shedding from sub-sea structures operate by modifying the boundary layer of the flow around the structure to prevent the correlation of vortex shedding along the length of the structure. Examples of such devices include sleeve-like devices such as helical strake elements, shrouds, fairings and substantially cylindrical sleeves. In general, such devices have a thickness aligned with the longitudinal axis of the cylinder of the length of a joint of the cylinder. Such thick devices often may be difficult to manufacture, transport, store, and/or install on the cylinder.
Some VIV and drag reduction devices can be installed on risers and similar structures before those structures are deployed underwater. Alternatively, VIV and drag reduction devices can be installed on structures after those structures are deployed underwater.
Elongated structures in wind in the atmosphere can also encounter VIV and drag, comparable to that encountered in aquatic environments. Likewise, elongated structures with excessive VIV and drag forces that extend far above the ground can be difficult, expensive and dangerous to install VIV and/or drag reduction devices.
U.S. Pat. No. 6,695,539 discloses apparatus and methods for remotely installing vortex-induced vibration (VIV) reduction and drag reduction devices on elongated structures in flowing fluid environments. The disclosed apparatus is a tool for transporting and installing the devices. The devices installed can include clamshell-shaped strake elements, shrouds, fairings, sleeves and flotation modules. U.S. Pat. No. 6,695,539 is herein incorporated by reference in its entirety.
Thus, there is a need in the art for an improved apparatus and method for suppressing VIV.
There is another need in the art for apparatus and methods for suppressing VIV which is easier to manufacture, transport, store, and/or install on a cylinder.
These and other needs of the present disclosure will become apparent to those of skill in the art upon review of this specification, including its drawings and claims.